The present invention relates to an electrophotographic imaging apparatus, and more particularly to systems and methods for characterizing laser beam process direction position errors.
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive drum by exposing select portions of the drum surface to laser light. Essentially, the density of the electrostatic charge on the surface of the drum is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the surface of the drum to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the drum surface in a manner that corresponds to the electrostatic density altered by the laser beam. Subsequently, a print medium such as paper is given an electrostatic charge opposite that of the toner and is pressed against the drum surface. As the medium passes the drum, the toner is pulled onto the surface thereof in a pattern corresponding to the latent image written to the drum surface. The medium then passes through a fuser that applies heat and pressure thereto. The heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
In order to produce an accurate representation of an image to be printed, it is necessary for the laser to write to the drum in a scan direction, which is defined by a straight line that is perpendicular to the direction of movement of the print media relative to the drum (the process direction). However, a number of optical elements including lenses and mirrors are typically required in the apparatus, including the printhead, to direct the laser beam towards the drum. Unavoidable imprecision in the shape and mounting of these optical elements with respect to the laser beam and/or drum can introduce process direction errors in the path of travel of the laser beam when writing across a scan line. It is also possible that a scan line written to the drum is not perpendicular to the movement of the print media due to laser misalignment and/or media misregistration. Under these conditions, there may be a skew associated with the printed image.
The prior art has attempted to correct for laser beam process direction position errors by incorporating carefully manufactured optics that are precisely aligned. However, the increased precision required by each optical element adds significantly to its cost. Even with precisely manufactured and aligned optics, the degree to which laser beam process direction position errors may be corrected is limited by several factors, including component tolerances. Moreover, distortion of the laser beam optical scan path can occur even in a precisely aligned system due to component aging and/or operational influences such as temperature changes.